<div class="view-child">
  <div class="box" #box>
    一个盒子
  </div>
  <div class="box" #box>
    2个盒子
  </div>
  <div class="box" #box>
    3个盒子
  </div>
  <div class="box" #box>
    4个盒子
  </div>
  <hr>
  <button class="btn btn-primary" (click)="showChildOne = !showChildOne">toggle child one</button>
  <app-panel></app-panel>
  <app-panel></app-panel>
  <app-panel></app-panel>
  <app-panel *ngIf="showChildOne"></app-panel>
  <app-panel></app-panel>
  <app-panel #panel></app-panel>
</div>